Moldable thermoplastic polyester crystalline resin blends offer a high degree of surface hardness, solvent resistance and abrasion resistance, high gloss, and low surface friction. However, loss of impact strength when subjected to ultra violet radiation may limit the usefulness of polyester crystalline resin blends for outdoor applications where molded articles made from the polyester will be exposed to sun and hot wet conditions.
Often a rubbery modifier is added to polyesters to improve impact strength. For example, improved impact strength is obtained by melt compounding polybutylene terephthalate with ethylene homo- and copolymers functionalized with either acid or ester moieties as taught in U.S. Pat. Nos. 3,405,198; 3,769,260; 4,327,764; and 4,364,280. Polyblends of polybutylene terephthalate with a styrene-alpha-olefin-styrene triblock are taught in U.S. Pat. No. 4,119,607. U.S. Pat. No. 4,172,859 teaches impact modification of polybutylene terephthalate with random ethylene-acrylate copolymers and EPDM rubbers grafted with a monomeric ester or acid functionality.
Although articles molded from impact-modified polyester resin/polycarbonate resin blends typically provide good impact performance, the weatherability of the such articles may be deficient in some applications where it is desired to retain the impact resistance after long term UV exposure. Hence, it is desirable to provide a molding composition having a combination of flame resistance, impact resistance with enhanced weatherability.
U.S. Pat. No. 4,161,469 describes a polymer blend comprising a polyalkyl terephthalate resin and organosiloxane-polycarbonate block copolymer having improved impact and heat distortion properties. U.S. Pat. No 4,794,141 describes polysiloxane/polycarbonate block copolymers, elastomeric polymers, and polyalkylene terephthalates. The elastomeric polymer is described as a hydrogenated block copolymer of a vinyl aromatic monomer and a conjugated diene. U.S. Pat. No. 5,380,795 describes a polymer mixture comprising an aromatic polycarbonate, a styrene-containing copolymer and/or graft polymer, and a polysiloxane-polycarbonate block copolymer, and articles formed therefrom. However, these patents do not describe flame-retarded blend and do not address question of weatherability of the blends.
U.S. Pat. No. 4,155,898 describes a polymer blend comprising a polyalkylene terephthalate, an organopolysiloxane-polycarbonate block copolymer, and a halogenated copolycarbonate having impact, heat distortion and flame retardant properties.
U.S. Pat. No. 5,981,661 describes a flame retarded molding compositions with enhanced weatherable properties, which comprise a polyester and polycarbonate blend with organopolysiloxane-polycarbonate block copolymer and a glycidyl ester impact modifier. However, high amount of gycidyl impact modifier could cause undesirable viscosity increase through the reaction between glycidy groups in the impact modifier and carboxyl groups in polyesters. In addition, gycidyl impact modifier is less effective impact modifiers than core-shell type rubbers. Accordingly, there is a need for enhancing the impact and processibility, as well as the retention of impact and color upon long term UV exposure. The present invention provides weatherable polyester crystalline resin blends with flame retardant properties for outdoor applications.